Options, options, options.....
1- Do a hard clean, don't worry about how much more damage that might occur, you know it's bad already.
2- Your pics only show what the outside damage is, but what about the inside.
3- Remove everything inside and float a hardwood or good plywood floor.
4- Seal and paint all - Cover all - Install a panel on the bad side and install a tank/tankless water heater on the good side.
Restorations are very expensive and you could be looking at double or triple the value. Need more pics on the slide to help. But again be innovated. Don't get caught up in original restoration... Good Luck Brother..